1. Technical Field
The invention relates to control of engine torque in a hybrid electric vehicle powertrain with an engine and an electric machine for developing motive power.
2. Background Art
A hybrid electric vehicle powertrain may include an internal combustion engine and an electric machine that function as complementary power sources for developing vehicle traction power. In one embodiment, a first power source is a combination of an engine with a gear system for distributing power through separate power flow paths. The engine may be a throttle-controlled internal combustion engine. The second power source is an electric drive system that comprises an electric motor, a generator and a battery. The battery acts as an energy storage medium for the generator and the motor. The generator provides electrical power to the motor, which delivers motive power to the traction wheels through the gear system. When the powertrain is operating using the first power source, the engine power is divided between the two paths by controlling generator speed.
A hybrid electric vehicle powertrain of this type is disclosed, for example in U.S. Pat. No. 7,275,518, which is assigned to the assignee of the present invention. The powertrain of the '518 patent acts in a manner that is similar to characteristics of a continuously variable transmission wherein vehicle speed changes do not depend upon engine speed. The description of the control system for the powertrain disclosed in U.S. Pat. No. 7,275,518 is incorporated herein by reference.
Since the engine and the generator in a powertrain of this type are connected through the gear system, the generator can act as a generator to provide current to charge the battery, which powers the motor, or it may operate as a motor to provide torque input to the gearing. The generator can be used to control the rotational speed of the engine since the generator is connected to a torque reaction element of the gearing. Both the motor and the generator may act as motors using current from the battery to provide the desired traction wheel torque. Alternatively, both the generator and the motor may act as generators to supply electrical power to the battery through a high voltage bus that electrically couples the motor, the generator and the battery.
In a hybrid electric vehicle of the type presently disclosed, the engine operates in a torque control mode rather than an engine speed control mode. Excess torque produced by the engine may develop charging current for the battery, which is controlled to manage battery performance. Further, a desired engine torque and a current engine torque are determined, and the difference can be adjusted based on a stored torque value. This implies that the engine torque made available to the traction wheels includes a stored torque offset, which can be adjusted within a certain engine speed range, based on a steady-state difference between the desired engine torque and the current engine torque to reduce the steady-state difference to zero in a closed loop fashion. If the engine throttle becomes contaminated with sludge, which may comprise environmental debris that affects the air flow around the periphery of a throttle blade, a significant engine torque error will exist between the desired engine torque and the actual engine torque. This adversely affects the calibration of the engine and the ability of the two power sources to function seamlessly to maintain a consistent and smooth torque delivery to the traction wheels from each power source as driver demand for traction wheel power changes.
If sludge accumulates in the throttle body of the engine in the disclosed embodiment of the invention, the actual torque may be less than the desired torque. This may cause the engine to stall in extreme cases because there then would be insufficient air for stable combustion. If there is compensation made for a decrease in the intake air for the throttle and a compensation for sludge is inaccurate, causing more air flow than would be desired for a given power demand, then the torque at the wheels will be greater than desired. This can result in poor drivability since the vehicle system controller would respond to the increased torque at the wheels by cutting fuel delivery to the engine in order to keep wheel torque within calibrated limits and to avoid overcharging the high voltage battery.